The present invention relates to a second harmonic generator for generating second harmonics from non-linear bifringent crystalline material of a resonator, and more particularly, to a second harmonic generator for attaining high-performance and enabling miniaturization by utilizing part of a generated second harmonic power to control the temperature of the non-linear bifringent crystalline material.
In general, a resonating beam is generated by means of a pair of mirrors located within a laser cavity, in order to improve the efficiency of a second harmonic generation.
A resonating beam structure is disclosed in U.S. Pat. No. 5,093,832. As shown in FIG. 1, the optical path of a reflected beam is split from the travelling path of an incident beam from a laser diode 12, within an optical resonator 20 having a pair of mirrors 22 and 26 provided at the respective ends thereof. Part of the output beam is deflected by a beam splitter 54, detected by a photo-detector 60 and fed back to peltier elements 80 through a temperature controller 70. However, in such a structure, since only about 90% of output beams is used for pick-up, the efficiency of beam utilization is relatively low. Also, using such a structure makes it difficult to miniaturize an SHG module since a feedback circuit having the photo-detector 60, differential amplifier 66, temperature controller 70, etc. is separately installed outside optical resonator 20. Moreover, beam quality may be lowered due to a mutual interference of the incident beams and reflected beams within beam splitter 54.
Also, U.S. Pat. No. 4,260,957, as shown in FIG. 2, discloses a method for extracting beams travelling reversely, among SHG beams bi-directionally generated from a non-linear bifringent crystalline material, potassium titanyl phosphate (KTP), for external transmission. In other words, in order to extract 100% of the reverse beam, another brewster plate 22 is applied, in addition to brewster plate 20. The angle of the plates is said to be the brewster angle against the fundamental frequency and is set to minimize the plates' interference with the fundamental frequency. However, since a separate brewster plate is additionally installed, the cost may be increased and the miniaturization of the SHG module is also difficult.